1. Field of the Invention
The present invention relates to a uni-morph, bi-morph or other type of piezoelectric or electrostrictive element as disclosed in co-pending U.S. patent application Ser. No. 07/912,920, which undergoes displacement in the form of bending, deflection or flexure and which can be suitably used for ink jet print heads, microphones, sounding bodies (such as loudspeakers), various resonators or vibrators, sensors, and other components or devices. The terms "piezoelectric element" and "electrostrictive element" used herein are interpreted to mean an element capable of transducing or converting an electric energy into a mechanical energy, i.e., mechanical force, displacement, strain or vibrations, or transducing a mechanical energy into an electric energy.
2. Discussion of the Related Art
In recent years, in the fields of optics and precision positioning or machining operations, for example, there have been widely used and increasingly demanded an element whose displacement is controlled for adjusting or controlling an optical path length or the position of a member or component of a device on the order of fractions of a micron (.mu.m), and a detecting element adapted to detect infinitesimal displacement of a subject as an electric change. To meet the needs, there have been developed or proposed various piezoelectric or electrostrictive actuators or sensors utilizing a piezoelectric or electrostrictive material such as a ferroelectric material, which exhibits the reverse or converse piezoelectric effect or the electrostrictive effect, in which such a piezoelectric or electrostrictive material produces a mechanical displacement upon application of a voltage or electric field thereto, or which exhibits the piezoelectric effect in which the piezoelectric material produces a voltage or electric field upon application of pressure or mechanical stress.
For example, an ink jet print head uses conventional piezoelectric/electrostrictive elements of a uni-morph or bi-morph type, so as to effect a printing operation utilizing displacement of the elements. In order to improve the quality and speed of the printing operation, efforts have been made to reduce the size of the piezoelectric/electrostrictive elements so as to form these elements with higher density, and lower the required drive voltage for displacing the elements, assuring improved operating response of the elements. In the piezoelectric or electrostrictive elements of modern vintage, a lower electrode, a piezoelectric/electrostrictive film and an upper electrode are laminated in the order of description on a ceramic substrate which serves as a vibration plate, by a film-forming method, so as to form a piezoelectric/electrostrictive unit or electrostrictive portion on the substrate. These piezoelectric/electrostrictive portion and substrate are then heat-treated into an integral laminar structure which serves as a piezoelectric/electrostrictive element.
In the film-type piezoelectric/electrostrictive element as described above, components contained in the layer of the ceramic substrate and the films of the electrode and piezoelectric/electrostrictive materials may react with those contained in the adjacent film or layer, at the interfaces of these layer and films, particularly at the interfaces between the electrode films and the piezoelectric/electrostrictive film, during heat treatment of the laminar piezoelectric/electrostrictive portion which consists of the electrode and piezoelectric-electrostrictive films. Such reaction may change the composition of the piezoelectric/electrostrictive film, resulting in changing the characteristics of the element. To achieve high stability or consistency in the characteristics of the piezoelectric/electrostrictive element, it is effective to remove a glass component or components from the materials of the piezoelectric/electrostrictive and electrode films, so as to restrict or reduce the reaction between the films, since the glass components are likely to react with a compound containing lead, which compound is included in the piezoelectric/electrostrictive material.
Although the removal of the glass component or components from the materials of the electrode and piezoelectric/electrostrictive films advantageously makes it possible to avoid a change of the composition of the piezoelectric/electrostrictive films, it also results in reduced bonding strength between the electrode films and piezoelectric/electrostrictive film, in particular, between the lower electrode and piezoelectric/ electrostrictive films. Consequently, these films may be separated from each other due to the insufficient bonding thereof, leading to shortened service life, poor operating characteristics and reduced operating reliability of the piezoelectric/electrostrictive element.
In the conventional piezoelectric/electrostrictive element, the piezoelectric/electrostrictive film is formed on a flat surface of the lower electrode film formed on the ceramic substrate. Accordingly, stresses are likely to act on the piezoelectric/electrostrictive film during the heat treatment thereof, whereby the characteristics of the element may deteriorate due to the residual stresses. Further, when a considerably large stress is applied to the piezoelectric/electrostrictive film, cracks or other defects may take place on the film, resulting in a reduced yield of the piezoelectric/electrostrictive elements produced.